1. Statement of the Technical Field
The present invention relates to the field of circulators and isolators, and more particularly to circulators and isolators that have variable RF properties.
2. Description of the Related Art
Circulators and isolators are devices that typically have three or more ports arranged in a ring and which provide unique RF transmission paths. An isolator is a three port circulator in which the third one of the ports has been terminated. Accordingly, for convenience, references to circulators herein shall be understood to also include isolators. Each type of device provides one way sequential transmission of power between its ports. For example, power in at port 1 couples only to port 2 with the exclusion of all other ports. More particularly, circulators and isolators are designed to allow RF energy to pass from a first port to a second port in a forward direction with little or no insertion loss, but present a high degree of attenuation for RF energy passing in a reversed direction from the second port to the first port. Similarly, RF energy is allowed to pass from the second port to a third port with low insertion loss, but is highly attenuated in the direction from the third port to the second port.
Circulators are often used to allow a receiver and a transmitter to share a common antenna by connecting a transmitter to port 1, an antenna to port 2 and a receiver to port 3. This arrangement provides for concurrent transmission and reception of signals. The antenna is always connected to the receiver and the transmitter but the receiver is isolated from the transmitted signals.
Most commonly, the fabrication of a circulator generally involves a three port Y junction of either rectangular waveguides or stripline that is loaded with ferrite cylinders or discs that are magnetized in a direction normal to the plane of the junction. Notably, while most circulators use a fixed direction of magnetic field and circulation, it is known in the art that the direction of circulation can be reversed by reversing the direction of the biasing magnetic field. This feature can be used to affect RF switching.
The ferrite discs used in circulators and isolators are typically formed from an iron powder that has been treated to produce an oxide layer on the outer surface. This oxide layer effectively insulates each iron particle from the next. The powder is mixed with a (non magnetic) ceramic bonding material and heated to form a rigid ceramic disc. Most common ferrite contains about 50% iron oxide. The remainder is typically either an oxide of manganese (Mn) and zinc (Zn) or nickel and zinc. Other types of ferrites can also be used to form the disc.
The operating frequency of circulators and isolators is primarily determined by the ferrimagnetic resonance frequency of the ferrite disk. The frequency of ferrimagnetic resonance can be affected by several factors including the diameter, permeability, and dielectric constant or permittivity of the ferrite disk. Maximum coupling of the energy from the RF signal to the ferrite material will occur at ferrimagnetic resonance. Accordingly, for reasons of efficiency, circulators and isolators are generally designed to operate either below ferrimagnetic resonance or above ferrimagnetic resonance. The operating frequency for below resonance (B/R) circulators are generally limited to the range from about 500 MHz to more than 30 GHz. By comparison, the practical range of operating frequencies for above resonance (A/R) circulators is lower, namely from about 50 MHz to approximately 2.5 GHz. From the foregoing, it may be observed that it can be difficult to design a single circulator capable of operating over a broad range of frequencies substantially below 500 MHz and more than 2.5 GHz.
Ferromagnetic materials (e.g. iron, nickel, cobalt, and various alloys) have atomic or molecular or crystalline magnetic dipole moments that exhibit a paramagnetic (i.e. positive feedback) response to magnetic fields. These dipole moments tend to align with the magnetic field but the alignment is disrupted by thermal motion of the atoms or molecules. In ferromagnetic materials, it is energetically favorable for all the dipole moments to be aligned. In at least some ferromagnetic materials, the field produced by the aligned dipoles is sufficient to maintain the alignment below the Curie temperature, thereby resulting in permanent magnets.
In ferrimagnetic materials, sometimes called ferrites, it is energetically favorable for neighboring dipole moments to be antiparallel but different types of atoms are present and the dipole moments do not cancel exactly. There can thus be a net positive dipole moment. Ferrimagnetic materials spontaneously subdivide into domains, small regions where all dipoles are parallel. The division into domains is such that total energy in the domain boundaries and fields is minimized. Arrangement of domains can be manipulated by externally applied electrical fields. It also influences the magnetic response of the material. These two properties are extremely useful in certain applications.